Conventional container coatings may be derived from a thermally curable formulation that includes particles of a thermoplastic material, typically vinyl chloride polymers (e.g., polyvinylchloride (PVC)), in an organic solvent. When these coatings are applied to a substrate and cured, the thermoplastic can degrade and discolor. To stabilize the thermoplastic material (i.e., reduce degradation) during the curing process, epoxy resins such as, for example, polyglycidyl ethers of cyclic polyols like bisphenol A (often referred to as “BADGE”) and epoxy novolacs, may be added to the coating formulation.
To reduce potential contamination of the packaged articles, it is desirable to reduce the extractable content of epoxy novolacs and other epoxy compounds in the coatings applied to food and beverage containers. Epoxy compounds with high epoxy equivalent weights may be used to reduce extractable epoxy content in the coating. However, if standard epoxies with equivalent weights higher than that of epoxy novolac (e.g., about 178) are incorporated in compositions at an effective level, coating performance is compromised, particularly adhesion. In addition, the hydroxyl functional groups in these high molecular weight compounds may react with additives such as secondary film formers (e.g., urea-formaldehyde resins, acrylics, and phenolics) in the coating formulation. This reaction increases crosslinking, which may decrease the adhesion and flexibility of the coating, thus limiting the use of the coating to only certain types of containers and applications. For example, a cured coating that is too brittle (e.g., from too much crosslinking) would be inappropriate for use in deeply drawn metal containers, which require sufficient coating flexibility to bend and form.
What is desired, yet not available in current coatings, is a stabilized coating composition that is sufficiently flexible, has high adhesion to metal, and is substantially free or free of epoxy novolacs.